Pipe Volume Formula & How to Calculate It
Same volume in every unit
Fluid weight
Estimated fill / drain time
Your recent calculations (saved on this device)
| When | ID | Length | Result |
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Use the calculator to verify the worked examples below at any bore, length or unit.
How to use this calculator
- Choose the unit system — US, Imperial or metric — at the top of the calculator.
- Pick the pipe type and schedule, then the nominal size, and the real inside diameter loads for you. Or choose Custom and type any bore.
- Enter the pipe length. Open Advanced to set fill level, number of identical runs, a fittings allowance, the fluid, or a flow rate.
- Press Calculate to see the volume in every unit, plus fluid weight, cross-section area and fill or drain time.
- Copy, share or print the result. Recent calculations are saved on your own device so you can reopen them later.
Tip: not sure where to start? Use the Fill example data button to load a complete worked example — a 2-inch Schedule 40 line, 50 ft long, at 10 gal/min — then adjust the numbers to match your job.
Real-world uses, with numbers
Charging a radiant-heat loop
How much water sits in a 300 ft run of ½" PEX.
≈ 2.9 US galSizing a well pressure tank
Line-fill volume in 100 ft of 1" Schedule 40.
≈ 4.5 US galFlushing a main
Water to displace in 100 ft of 4" Schedule 40.
≈ 66 gal · 250 LWater weight on hangers
Load from a full 100 ft run of 2" pipe.
≈ 145 lb · 66 kgMetric heating fill
Charge volume for 50 m of 28 mm bore.
≈ 30.8 LFuel or oil line contents
Diesel held in 20 m of 50 mm bore.
≈ 39 L · 33 kgMore questions, answered
How do I convert between gallons, litres and cubic metres?
1 US gallon = 3.785 litres; 1 Imperial gallon = 4.546 litres; 1 m³ = 1,000 litres = 264.2 US gallons. The calculator shows all of them from a single entry, so you never convert by hand.
Does the pipe material change the volume?
Only through the bore. Two pipes with the same inside diameter hold the same volume whether they are steel, PVC, copper or PEX. Material and schedule matter because they set the real inside diameter for a given nominal size.
How much does the water inside a pipe weigh?
About 8.34 lb per US gallon, or 1 kg per litre. A full 100 ft run of 2" Schedule 40 holds roughly 17.4 gallons, about 145 lb. Set the fluid in Advanced for hot water, glycol, diesel or seawater.
What if I only know the outside diameter?
Switch the diameter to OD mode and enter the wall thickness; the tool subtracts two wall thicknesses to reach the bore. For a listed standard, just pick the size and the published inside diameter loads automatically.
Can I use it for a part-full or sloped drain?
Yes for a straight, round pipe: open Advanced and set the fill level below 100%, and the volume scales with it. Truly partial gravity flow in a sloped line follows a curved free surface, so treat the part-full figure as an approximation.
Notes and derived figures
- Volume scales with the square of the bore: doubling the inside diameter roughly quadruples the capacity, while doubling the length only doubles it.
- Per-foot and per-metre capacity are constant for a given bore, so any length is a single multiplication — handy for quick field estimates.
- Nominal size is a label. A "2-inch" Schedule 40 pipe has a 2.067-inch bore; using "2" overstates the volume by more than a third.
- Fill and drain time is volume divided by flow rate. Enter a flow rate in Advanced to turn any result into a charge, flush or drain time.
The pipe volume formula
A pipe is a cylinder, so its internal volume is the area of the circular bore multiplied by the length:
V = π × r² × L = π × (ID / 2)² × L
r is the internal radius, ID the internal diameter, L the length. π ≈ 3.14159.
Two rules make it reliable. First, use the internal diameter, never the nominal size or outside diameter. Second, keep the diameter and length in the same unit before you multiply, then convert the volume to whatever unit you want at the end.
How to calculate pipe volume, step by step
- Get the bore. Look up or measure the internal diameter. For standard pipe, use a schedule chart — a 3-inch Schedule 40 pipe has a 3.068 in bore, not 3 in.
- Halve it. The radius is ID ÷ 2.
- Square and multiply by π. That is the cross-sectional area.
- Multiply by the length (in the same unit) to get the raw volume.
- Convert to gallons, litres or cubic feet as needed.
Worked example — gallons
Find the volume of 50 ft of 3-inch Schedule 40 pipe in US gallons.
- Bore = 3.068 in, so radius = 1.534 in.
- Area = π × 1.534² = 7.393 in².
- Length = 50 ft = 600 in; volume = 7.393 × 600 = 4,436 in³.
- Gallons = 4,436 ÷ 231 = 19.2 US gallons.
Worked example — litres
Find the volume of 20 m of DN80 (3-inch) pipe with an 80 mm bore.
- Radius = 0.040 m; area = π × 0.040² = 0.005027 m².
- Volume = 0.005027 × 20 = 0.1005 m³.
- Litres = 0.1005 × 1000 = 100.5 litres.
Common mistakes to avoid
- Using the nominal size. It is a label, not a measurement.
- Using the outside diameter. The wall does not hold fluid.
- Mixing units. Convert the diameter to the length's unit first.
- Confusing US and Imperial gallons. They differ by about 20%.
Verify with the calculator
Enter the same numbers in the calculator above and you should get the same answers — 19.2 US gallons and 100.5 litres for the two examples. Once you trust the method, the tool is simply the fast way to apply it across sizes, materials and fill levels without arithmetic slips.
Reading and trusting your result
Here is how to read the number this pipe volume formula gives you, and where it stops being exact.
What the figure represents
The headline value is the internal capacity of a round, uniform pipe running full — the fluid the bore can hold along the length you entered. Every other unit on the panel is the same physical quantity re-expressed, so you can quote whichever your paperwork needs without recalculating.
The full-pipe assumption
By default the figure assumes the pipe is completely full and perfectly cylindrical. For gravity drains and vented lines that run part full, set a fill level below 100% and the result scales with it. Bends, valves and fittings add a little capacity the straight-length model does not see — use the fittings allowance to approximate them.
Capacity reference tables & a worked example
Concrete numbers for this page, calculated the same way the tool above does — use them as a quick check or a lookup.
Schedule 40 2" pipe: volume by run length
How much a 2" Schedule 40 bore holds as the run gets longer, in US gal. The per-foot figure is constant, so any length is one multiplication away.
| Length (ft) | Volume (US gal) | Per foot |
|---|---|---|
| 10 ft | 1.74 | 0.1743 |
| 25 ft | 4.36 | 0.1743 |
| 50 ft | 8.72 | 0.1743 |
| 100 ft | 17.43 | 0.1743 |
| 250 ft | 43.58 | 0.1743 |
| 500 ft | 87.16 | 0.1743 |
Schedule 40 pipe: volume across sizes
Volume held by a fixed 100 ft run across common Schedule 40 sizes, in US gal. Doubling the diameter roughly quadruples the capacity.
| Nominal size | Bore (in) | Volume in 100 ft (US gal) |
|---|---|---|
| 1/2" | 0.622 | 1.58 |
| 3/4" | 0.824 | 2.77 |
| 1" | 1.049 | 4.49 |
| 1-1/2" | 1.610 | 10.58 |
| 2" | 2.067 | 17.43 |
| 3" | 3.068 | 38.40 |
| 4" | 4.026 | 66.13 |
| 6" | 6.065 | 150.1 |
Worked example, step by step
- Bore = 2.067 in, so radius = 1.0335 in.
- Area = π × radius² = 3.3556 in².
- Length = 75 ft = 900 in; volume = 3.3556 × 900 = 3,020.0 in³.
- Convert to the unit you need: 13.07 US gal for a 75 ft run of Schedule 40 2" pipe.
Frequently asked questions
What is the formula for the volume of a pipe?
V = π × (ID/2)² × L, where ID is the internal diameter and L the length. Keep both in the same unit before multiplying.
How do you calculate the volume of a pipe in gallons?
Work out the volume in cubic inches (area in in² × length in inches), then divide by 231 for US gallons or 277.4 for Imperial gallons.
Do I use radius or diameter in the pipe volume formula?
Either, as long as you are consistent: r² with the radius, or (ID/2)² with the diameter. Both give the same area.
Why can't I just use the pipe's nominal size?
Nominal size is a naming label. The real bore is usually smaller, so using the nominal number overstates the volume — sometimes by a third or more.
Related pipe volume tools
What people say
Used the litres-per-metre figures to size a glycol charge for a radiant job. Numbers matched the datasheet.
Switches between US and metric without losing the pipe. Small thing, big time-saver.
Would love more HDPE DIPS sizes, otherwise excellent.